1. Field of the Invention
The present invention relates to new fluorine-containing blocked polyisocyanates, their preparation and their use for the preparation of polyurethane plastics, preferably as crosslinking components for stoving lacquers, in particular for coil coatings.
2. Description of the Prior Art
The formulation of blocked polyisocyanates with OH-containing polycondensates or polymers (polyesters or polyacrylates) to give binders for xe2x80x9cone-componentxe2x80x9d stoving lacquers or stoving coating compositions is known. The incorporation of fluorine into lacquer binders for the purpose of achieving a particular water- and soil-repellent coating is also prior art.
There are various publications in the patent literature in which fluorine is mentioned as a modifying component either in the OH or in the NCO component of coating compositions.
Blocked polyisocyanates with a content of incorporated fluorine are described, for example, in the U.S. Pat. Nos. 5,541,281 and 5,576,411. These are polyisocyanates with allophanate, isocyanurate and urethane groups which are prepared by the reaction of fluorine-substituted alcohols and monomeric diisocyanates. It is understood that the preparation of fluorine-containing polyisocyanates from the base isocyanates is a relatively expensive process, inter alia, because of the thin film distillations to be carried out to remove the starting isocyanates (purification). Also, it must be taken into account that such fluorine-containing isocyanates have a limited field of use and are therefore so-called niche products.
It is an object of the present invention to provide an alternative solution for the preparation of fluorine-containing polyisocyanates. An alternative to the known route is the modification of commercially available lacquer polyisocyanates with fluoro-alcohols. It is an additional object of the present invention to provide fluorine-containing blocked polyisocyanates from conventional lacquer polyisocyanates by a simple process, which can be employed for the preparation of polyurethane plastics, preferably stoving lacquers with a water- and soil-repellent surface.
This object may be achieved with the blocked polyisocyanates according to the invention.
The present invention relates to blocked fluorine-containing polyisocyanates which have a fluorine content, calculated as F=19, of 1.0 to 20.0 wt. %, preferably 4.0 to 10.0 wt. %, and are suitable for preparing stoving coatings having a water- and soil-repellent surface, wherein the fluorine-containing polyisocyanates are based on the reaction product of aliphatic polyisocyanates or polyisocyanate mixtures having an NCO content of 10 to 25 wt. % and a functionality of at least 2.5 with monofunctional isocyanate blocking agents and fluorinated monoalcohols wherein
i) 75 to 95 equivalent-% of the isocyanate groups are reacted with isocyanate blocking agents,
ii) 5 to 25 equivalent-% of the isocyanate groups are reacted with fluorinated monoalcohols and
iii) the equivalents of i) and ii) add up to 100%.
The present invention also relates to a process for the preparation of blocked fluorine-containing polyisocyanates by initially introducing polyisocyanates, optionally in a solvent, into a reaction vessel at a temperature of 50 to 70xc2x0 C. with stirring, then adding a monofunctional fluorine-substituted alcohol and carrying out the reaction at 80 to 110xc2x0 C. until the calculated NCO content is obtained, and then reacting the remaining NCO groups with the corresponding amount of blocking agent at 70 to 100xc2x0 C. until NCO groups are then no longer detected, e.g. by means of the IR spectrum.
The present invention additionally relates to the use of the fluorine-containing blocked polyisocyanate according to the invention as a crosslinking agent in the preparation of polyurethane plastics.
The present invention finally relates to substrates which are coated with lacquers containing the blocked fluorine-containing polyisocyanates according to the invention.